Syncthing is great, but it's peer to peer, and it requires block storage ("filesystem"). It also has no idea of point-in-time snapshots. Syncthing, is, as is unison, rsync, etc, basically a mirroring tool.
By contrast, Borg, Restic, Kopia (anything else?) use object storage, aka binary blobs, like S3 or R2 or One Drive. They store both entire copies and small diffs on top of them, much like video codecs, or like git. You can look at the filesystem you've backed up as it was in a particular moment, and you may have a history of many such moments, say, daily snapshots for a month, stored economically, not as 30 full copies. And it all is encrypted on top. If your source FS supports snapshots (ZFS, XFS on LVM, BTRFS), your backups can be entirely consistent views of your filesystem, of its relevant subtrees.